Prior power control circuits for typical applications such as motor speed control, incandescent lamp dimmers and the like generally employed thyristors such as triacs or silicon controlled rectifiers in series between an AC power source and the load. Assuming a sixty cycle/sec AC power source, current in the load passed through zero one hundred twenty times per second. Each time the load current passed through zero, the thyristor turned off.
An electronic timing circuit triggered the thyristor to turn on, i.e. established the thyristor firing angle, and thereby controlled the power to the load. If the timing circuit provided for no delay, full power was applied to the load. As the time delay increased, the load received less and less power.
Since just prior to thyristor turn-on, no current was delivered to the load and a relatively large current flow occurred immediately after thyristor turn-on, radio frequency interference resulted from the current transient.
One approach at reducing such radio frequency interference was the employment of an inductor in series with the load, as mentioned in U.S. Pat. No. 3,793,557 issued to CRAMER. The CRAMER patent disclosed a gapped core inductor with at least two air gaps in series with the thyristor and the load.